A radio (wireless) communication system may be comprised of an access network and a plurality of access terminals. The access network may include access points, such as Node Bs, base stations, or the like, that allow the access terminals to connect with the access network for uplink (UL: terminal-to-network) communications and downlink (DL: network-to-terminal) communications via various types of channels. The access terminals may be user equipment (UE), mobile stations, or the like.
Although the concepts described hereafter may be applicable to different types of communication systems, the Universal Mobile Telecommunications System (UMTS) will be described merely for exemplary purposes. A typical UMTS has at least one core network (CN) connected with at least one UTRAN (UMTS Terrestrial Radio Access Network) that has Node Bs acting as access points for multiple UEs.
FIG. 1 shows the radio interface protocol architecture according to the 3GPP radio access network standards. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data and a control plane (C-plane) for transmitting control information. The user plane is a region that handles traffic information with the user, such as voice or Internet protocol (IP) packets. The control plane is a region that handles control information for an interface with a network, maintenance and management of a call, and the like.
The protocol layers in FIG. 1 can be divided into a first layer (L1), a second layer (L2), and a third layer (L3) based on the three lower layers of an open system inter-connection (OSI) standard model. The first layer (L1), namely, the physical layer (PHY), provides an information transfer service to an upper layer by using various radio transmission techniques. The physical layer is connected to an upper layer called a medium access control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. The second layer (L2) includes a MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer. The MAC layer handles mapping between logical channels and transport channels and provides allocation of the MAC parameters for allocation and re-allocation of radio resources. The MAC layer is connected to an upper layer called the radio link control (RLC) layer, via a logical channel. Various logical channels are provided according to the type of information transmitted.
The MAC layer is connected to the physical layer by transport channels and can be divided into a MAC-b sub-layer, a MAC-d sub-layer, a MAC-c/sh sub-layer, a MAC-hs sub-layer and a MAC-m sublayer according to the type of transport channel being managed. The MAC-b sub-layer manages a BCH (Broadcast Channel), which is a transport channel handling the broadcasting of system information. The MAC-c/sh sub-layer manages a common transport channel, such as a forward access channel (FACH) or a downlink shared channel (DSCH), which is shared by a plurality of terminals, or in the uplink, the Random Access Channel (RACH). The MAC-m sublayer may handle the MBMS data. The MAC-d sub-layer manages a dedicated channel (DCH), which is a dedicated transport channel for a specific terminal. The MAC-d sublayer is located in a serving RNC (SRNC) that manages a corresponding terminal and one MAC-d sublayer also exists in each terminal.
The RLC layer, depending of the RLC mode of operation, supports reliable data transmissions and performs segmentation and concatenation on a plurality of RLC service data units (SDUs) delivered from an upper layer. When the RLC layer receives the RLC SDUs from the upper layer, the RLC layer adjusts the size of each RLC SDU in an appropriate manner based upon processing capacity, and then creates data units by adding header information thereto. These data units, called protocol data units (PDUs), are transferred to the MAC layer via a logical channel. The RLC layer includes a RLC buffer for storing the RLC SDUs and/or the RLC PDUs.
The BMC layer schedules a cell broadcast (CB) message transferred from the core network and broadcasts the CB message to terminals positioned in a specific cell or cells.
The PDCP layer is located above the RLC layer. The PDCP layer is used to transmit network protocol data, such as IPv4 or IPv6, efficiently on a radio interface with a relatively small bandwidth. For this purpose, the PDCP layer reduces unnecessary control information used in a wired network, namely, a function called header compression is performed.
The radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane. The RRC layer controls the transport channels and the physical channels in relation to setup, reconfiguration, and the release or cancellation of the radio bearers (RBs). The RB signifies a service provided by the second layer (L2) for data transmission between the terminal and the UTRAN. In general, the set up of the RB refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific data service, and setting the respective detailed parameters and operation methods. Additionally, the RRC layer handles user mobility within the RAN, and additional services, e.g., location services.
Call establishment between a UE (User Equipment) and a radio network in current wireless communications systems, such as UMTS, is performed on a RACH (Random Access Channel) according to appropriate procedures. The timing at which the UE can start a random access procedure is derived on the basis of an Access Service Class (ASC) that gives a priority level for access attempts. The random access procedure is divided into two phases: an access attempt phase, and when the access is succeeded the message transmission phase indicating an establishment cause. When the establishment cause is decoded by the network, depending upon the request and the radio resource availability, a decision is made by the network to accept or to reject the call establishment.
In general, the procedure where the UE sends a first message to the network is referred to as initial access. For this, the common uplink channel called RACH (Random Access Channel) is used. In all cases (GSM and UMTS systems), the initial access starts from the UE with the connection request message including the reason of the request, and the answer from the network indicating the allocation of radio resources for the requested reason.
There are several reasons, which may be referred to as an establishment cause, for sending a connection request message and the following list shows some examples specified in UMTS:
Originating Conversational Call,
Originating Streaming Call,
Originating Interactive Call,
Originating Background Call,
Originating Subscribed traffic Call,
Terminating Conversational Call,
Terminating Streaming Call,
Terminating Interactive Call,
Terminating Background Call,
Emergency Call,
Inter-RAT cell re-selection,
Inter-RAT cell change order,
Registration, Detach,
Originating High Priority Signalling,
Originating Low Priority Signalling,
Call re-establishment,
Terminating High Priority Signalling,
Terminating Low Priority Signalling,
Regarding the definitions of the terms used above, originating call means that the UE wants to setup a connection (for instance a speech connection), terminating call means that the UE answers to paging, while registration means that the user wants to register only to perform a location update.
To send the information over the air interface, the physical random access procedure is used. The physical random access transmission is performed under the control of a higher layer protocol, which performs some important functions related to priority and load control. These procedures differ between GSM and UMTS radio systems. The description of GSM random access procedure can be found in The GSM System for Mobile Communications, published by M. Mouly and M. B. Pautet, 1992. As the present innovation is UMTS enhancement/evolution related, the W-CDMA random access procedure will be described in more detailed below.
In the UMTS physical layer random access procedure, the UE randomly selects an access resource and transmits a RACH preamble part of a random access procedure to the network. The preamble is a short signal that is sent before the transmission of the RACH connection request message. The UE repeatedly transmits the preamble by increasing the transmission power every time the preamble is sent until it receives an AI (Acquisition Indicator) on an AICH (Acquisition Indicator Channel), which indicates the detection of the preamble by the network. The UE stops the transmission of the preamble once it receives the AI (Acquisition Indicator), and sends the message part at a level of power equal to the preamble transmission power at that point, plus an offset signalled by the network. This random access procedure avoids a power ramping procedure for the entire message. Such power ramping procedure would create more interference due to unsuccessful sent messages and it would be less efficient due to the larger delay, since it would take much more time to decode the message before an acknowledgement could be given that it was received successful.
The main characteristics of the RACH is that it is a contention based channel, which means that due to simultaneous access of several users, collisions may occur such that the initial access message cannot be decoded by the network. The UE can start the random-access transmission (both preambles and message) at the beginning of an access slot only. This kind of access method is therefore a type of slotted ALOHA approach with fast acquisition indication.
FIG. 2 shows an example of the timing (i.e. access slots) related to a random access transmission, while FIG. 3 shows examples of the reception of a downlink AICH access slot by the UE and the reception of an uplink PRACH access slot by the UE.
The time axis of both the RACH and the AICH is divided into time intervals, called access slots. There are 15 access slots per two frames (one frame is 10 ms in length or 38400 chips) and they are spaced 1.33 ms (5120 chips) apart. Information on what access slots are available for random-access transmission and what timing offsets to use between RACH and AICH, between two successive preambles and between the last preamble and the message is signalled by the network. If the AICH transmission timing is 0 and 1, it is sent 3 and 4 access slots after the last preamble access slot transmitted, respectively
As for the format of the preambles, each preamble consists of 4096 chips, which is a sequence of 256 repetitions of Hadamard codes of length 16. The Hadamard codes are referred to as the signature of the preamble. There are 16 different signatures and a signature is randomly selected (from available signatures sets on the basis of ASC) and repeated 256 times for each transmission of the preamble part.
FIG. 4 shows an exemplary structure (format) of the AICH. The AICH consists of a repeated sequence of 15 consecutive access slots, each having a length of 40 bit intervals (5120 chips). Each access slot consists of two parts, an Acquisition Indicator (AI) part consisting of 32 real-valued signals a0, . . . , a31 and a part of duration 1024 chips where transmission is switched off.
When the network detects the transmission of a RACH preamble in a RACH access slot with a certain signature, it repeats this signature in the associated AICH access slot. This means that the Hadamard code used as the signature on the RACH preamble is modulated onto the AI part of the AICH. The acquisition indicator corresponding to the signature can take the values +1, −1, and 0, depending upon whether a positive acknowledgement, a negative acknowledgement or no acknowledgement is given to a specific signature.
A positive polarity of the signature indicates that the preamble has been acquired and the message can be sent. A negative polarity indicates that the preamble has been acquired and the power ramping procedure shall be stopped, but the message shall not be sent. This negative acknowledgement is used when there is a congestion situation in the network, and thus a transmitted message cannot be processed at the present time. In this case, the access attempt needs to be repeated some time later by the UE.
For the control of random access transmissions, the network decides whether the mobile station should be permitted to use radio access resource based primarily upon the access class to which the UE belongs. A specified priority level is implied by the Access Class (AC) which is stored on the UE SIM card.
Certain aspect of access control will be described hereafter. Regarding the purpose of access control, under certain circumstances, it will be desirable to prevent UE users from making access attempts (including emergency call attempts) or responding to pages in specified areas of a PLMN (Public Land Mobile Network). Such situations may arise during states of emergency, or where 1 of 2 or more co-located PLMNs has failed. Broadcast messages should be available on a cell-by-cell basis indicating the class(es) of subscribers barred from network access. The use of this facility allows the network operator to prevent overload of the access channel under critical conditions. It is not intended that access control be used under normal operating conditions.
FIG. 5 shows an example of the types of access classes (AC) and their respectively related access service (AS), each containing an information element (IE).
For allocation, all UEs are members of one out of ten randomly allocated mobile populations, defined as Access Classes 0 to 9. The population number can be stored in a SIM/USIM for the UE. In addition, the UEs may be members of one or more out of 5 special categories (Access Classes 11 to 15), which also may be stored in the SIM/USIM. These may be allocated to specific high priority users as follows. (This enumeration is not meant as a priority sequence):
Class 15—PLMN Staff;
Class 14—Emergency Services;
Class 13—Public Utilities (e.g. water/gas suppliers);
Class 12—Security Services;
Class 11—For PLMN Use.
For operation, if the UE is a member of at least one Access Class which corresponds to the permitted classes as signalled over the air interface, and the Access Class is applicable in the serving network, access attempts are allowed. Otherwise access attempts are not allowed.
Access Classes are applicable as follows:
Classes 0˜9—Home and Visited PLMNs;
Classes 11 and 15—Home PLMN only;
Classes 12, 13, 14—Home PLMN and visited PLMNs of home country only.
Any number of these classes may be barred at any one time.
For emergency calls, an additional control bit known as Access Class 10 is also signalled over the air interface to the UE. This indicates whether or not network access for Emergency Calls is allowed for UEs with access classes 0 to 9 or without an IMSI. For UEs with access classes 11 to 15, Emergency Calls are not allowed if both Access Class 10 and the relevant Access Class (11 to 15) are barred. Otherwise, Emergency Calls may be allowed.
In UMTS, the AC are mapped to Access Service Classes (ASC). There are eight different priority levels defined (ASC 0 to ASC 7), with level 0 being the highest priority.
For Mapping of Access Classes to Access Service Classes, the Access Classes shall only be applied at initial access, i.e. when sending an RRC CONNECTION REQUEST message. A mapping between Access Class (AC) and Access Service Class (ASC) shall be indicated by the information element AC-to-ASC mapping in System Information Block type 5. The correspondence between AC and ASC is indicated in FIG. 5.
In the table of FIG. 5, the nth IE designates an ASC number i in the range 0-7 to AC. If the ASC indicated by the nth IE is undefined, the UE behaviour is unspecified.
For random access, the parameters implied by the respective ASC shall be employed. In case the UE is a member of several ACs, it shall select the ASC for the highest AC number. In connected mode, AC shall not be applied.
An ASC consists of a subset of RACH preamble signatures and access slots, which are allowed to be used for this access attempt and a persistence value corresponding to a probability Pv≦1 to attempt a transmission. Another mechanism to control random access transmission is a load control mechanism, which allows reducing of the load of incoming traffic when the collision probability is high or when radio resources are low.
FIGS. 6 and 7 show a flow chart of the control access procedure.
1. Existing specifications provide many RACH transmission control parameters which are stored and updated by the UE based on system information broadcasted by the network. The RACH transmission control parameters include Physical RACH (PRACH), Access Service Class (ASC), a maximum number of preamble ramping cycles Mmax range of backoff interval for timer TBO1 given in terms of numbers of transmission 10 ms time intervals NBO1max and NBO1min, applicable when negative acknowledgement on AICH is received (S201).
2. The UE maps the assigned AC to an ASC, and a count value M is set to zero (S203 S205, S207).
3. The count value M is incremented by one (S209). Next, the UE determines if the count value M representing the number of transmission attempts exceeds the maximum number of permitted RACH transmission attempts Mmax (S211). If so, then the UE treats the transmission as unsuccessful (S212).
4. However, if M is less than or equal to the maximum number of permitted RACH transmission attempts Mmax, then the UE updates the RACH transmission control parameters (S213). In the next step, a 10 ms timer T2 is set (S215). The UE decides whether to attempt transmission based on the persistence value Pi associated with the ASC selected by the UE. Specifically, a random number Ri is generated between 0 and 1 (S217). If the random number Ri is less than or equal to the persistence value Pi, the UE attempts to transmit over the assigned RACH resources, otherwise, the UE waits until the 10 ms timer T2 expires and perform the procedure in step 4 again (S219, S220, S221).
5. When one access attempt is transmitted, the UE determines whether the network responds with an Acknowledgement (ACK), a Non Acknowledgment (NACK), or no response (S223). If no response is received from the network, after the timer T2 expires the process is performed again from step 3 (S224). If a NACK, indicating a failed receipt of the transmission by the network (often due to collision) is received, then the UE waits for the timer T2 to expire then generates a back off value NBO1 randomly chosen between the maximum and minimum back off values NBO1max and NBO1min associated with the PRACH assigned to the UE (S225). The UE then waits a back off interval TBO1 equal to 10 ms times the back off value NBO1 before performing the process from step again (S226). If an ACK, indicating receipt of the UE transmission by the network, is received, then the UE begins the message transmission (S227).
FIG. 8 shows an example of Signalling Establishment procedure. Once the PRACH power control preambles have been acknowledged, the RRC Connection Request message can be transmitted (S81). It contains the reason why the connection is requested.
Depending on the request reason, the radio network makes a decision regarding the kind of resources to reserve, and performs synchronization and signaling establishment among certain radio network nodes (i.e. Node B and serving RNC) (S82). When the radio network is ready, it sends to the UE the Connection Setup message conveying information about the radio resources to use (S83). The UE confirms connection establishment by sending the Connection Setup Complete message (S84). When the connection has been established, the UE sends an Initial Direct Transfer message including a large amount of information, such as the UE identity, its current location, and the kind of transaction requested (S85). Then the UE and the network authenticate each other and establish security mode communication (S86). The actual set up information is delivered through the Call Control Setup message (S87). It identifies the transaction and indicates the quality of service (QoS) requirements. Upon receiving the message, the network starts activities for radio bearer allocation by checking if there are enough resources available to satisfy the requested QoS. If yes, the radio bearer is allocated according to the request. If not, the network may select either to continue allocation with a lowered QoS value, or it may select to queue the request until radio resources become available or to reject the call request (S88, S89).